In the related Application, we described a method and network that enable highly reliable communication between devices (transceivers). Along with reliability, the latency of communications between devices is also a key performance metric for many control and monitoring applications.
Typically, the network has a star topology. A master device (master) transmits commands, and receives status updates from slave devices (slaves).
In a star network, communications from the master to the slaves use downlink (DL) channel, and communications from the slaves to the master use an uplink (UL) channel. The time interval from the beginning of a broadcast polling message to the next polling message is called a superframe. The superframe interval is 10 ms to limit an overall response time from each slave to be less than 10 ms. There are four types of packets: broadcast polling, slave response, group acknowledgment (GACK) and response retransmission (Retx).
The superframe and packets are described in complete detail in the related Application, incorporated herein by reference.
Superframe
The superframe has two retransmission intervals. The Broadcast polling and GACK packets are on the DL channels, and the responses and Retx packets are on the uplink channel.
In wireless networks, the effects of multipath can cause severe degradation of the signal to noise ratio (SNR) due to destructive self interference of the echoes of the transmitted signal at the receiver. Orthogonal Frequency Division Modulation (OFDM) is a common method to decrease the multipath effects. Essentially, data are transmitted in a parallel fashion on a set of subcarriers, effectively transforming the wireless channel into a set of parallel communication channels. Each channel sees a time varying attenuation and phase rotation.
OFDM is specified by a number of wireless standards such as IEEE 802.11g, 802.11n and WiMAX. In the OFDM network, the bandwidth available for communications can be considered as a set of N parallel channels or subcarrier.
A label for each subcarrier is a physical index. As defined herein, a carrier (or subcarrier) is a frequency band. Each of these subcarriers is used to transmit a single modulation symbol which can be drawn from any modulation constellation. Binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16 QAM) or 64 QAM are some examples of digital modulation formats that can be used.
The advantage of the OFDM network is that each subcarrier has a narrow bandwidth so that it receives on a simple flat fading channel. Additionally, one can transmit the same data (modulation symbols) over several subcarriers, and thus, obtain a frequency diversity gain. This assumes that the subcarriers have a frequency separation that is at least as large as the channel coherence bandwidth. Then, the receiver can perform a maximal ratio combining of the signals on each of the subcarriers to improve the received signal to noise ratio.